EGR control device and method for internal combustion engine

ABSTRACT

An EGR control device is provided with an EGR control valve ( 52 ) and an electronic control unit ( 60 ). An exhaust gas recirculation pipe ( 51 ) extends across the EGR control valve ( 52 ). The electronic control unit calculates a target EGR ratio, a target air flow rate, an actual EGR ratio, and an actual air flow rate, on the basis of operational state quantities of an engine. The electronic control unit then calculates, as a target converted EGR ratio, a ratio of the target EGR ratio to the target air flow rate, calculates, as an actual converted EGR ratio, a ratio of the actual EGR ratio to the actual air flow rate, and controls an opening of the EGR control valve such that the target converted EGR ratio becomes equal to the actual converted EGR ratio. An actual converted EGR ratio is proportional to an intake-air oxygen concentration, and an actual converted EGR ratio and a target converted EGR ratio are calculated independently of a command injection amount. Therefore, a desired intake-air oxygen concentration is obtained irrespective of flow rate characteristics of injection valves.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an EGR control device and method for aninternal combustion engine.

2. Description of the Related Art

There has been known an EGR unit (an exhaust gas recirculation unit)that recirculates part of exhaust gas of an internal combustion engineinto an intake passage. Such an EGR unit, which is designed to reduce anamount of nitrogen oxides (NOx) and the like that are discharged from anengine, is provided with an EGR gas passage through which an exhaustpassage and an intake passage of the engine communicate with each other,and with an EGR control valve across which the EGR passage extends. Bycontrolling an opening of the EGR control valve, a flow rate of EGR gasis controlled. Thus, an EGR ratio, which is a ratio of a flow rate ofEGR gas sucked by the engine to a flow rate of gas sucked by the engine(cylinders) (i.e., a flow rate of the entire gas), is controlled.

An amount of NOx discharged from a diesel engine is closely correlatedto a concentration of oxygen contained in gas flowing into cylinders ofthe engine (hereinafter referred to as an “intake-air oxygenconcentration “x””). Accordingly, in order to reduce a discharge amountof NOx, it is effective to perform control such that the intake-airoxygen concentration “x” becomes equal to a predetermined concentration.In view of the foregoing, an internal combustion engine equipped with anEGR unit disclosed in Japanese Patent Application No. 10-141147 adopts aconcept of converted EGR ratio so as to control an intake-air oxygenconcentration “x”, and controls an EGR control valve such that an actualconverted EGR ratio becomes equal to a target converted EGR ratio.

That is, in the unit disclosed in Japanese Patent Application Laid-OpenNo. 10-141147 mentioned above, an air excessiveness ratio λ is definedas expressed by an expression (1) shown below. A converted EGR ratio SRis defined as a value obtained by dividing an EGR ratio R by the airexcessiveness ratio λ (SR=R/λ). As is apparent from FIG. 5, there isestablished a relationship approximately expressed by an expression (2)shown below between the converted EGR ratio SR and the intake-air oxygenconcentration “x”.

[Expression 1]λ=k×Gn/Q  (1)

k . . . constant

Gn . . . flow rate of air (fresh air) that is newly sucked into engine(cylinders)

Q . . . fuel injection amount per unit time

[Expression 2]SR=R/λ≈p×x+q  (2)

p: negative constant, q: positive constant

Thus, as is understood from the aforementioned expression (2), theintake-air oxygen concentration “x” can be made equal to a suitableconcentration by determining a target converted EGR ratio SRtgt as atarget value of the converted EGR ratio SR on the basis of operationalstate quantities (e.g., a command injection amount Qfin and an enginerotational speed NE), calculating a true converted EGR ratio (an actualconverted EGR ratio) SRact on the basis of the operational statequantities of the engine, and controlling an opening of the EGR controlvalve such that the actual converted EGR ratio SRact coincides with thetarget converted EGR ratio SRtgt. The aforementioned unit of the relatedart controls EGR ratio on the basis of the concept as described above,thus reducing a discharge amount of NOx.

In this case, the target converted EGR ratio SRtgt is determined, forexample, on the basis of a map (a table) defining a relationship betweencommand injection amount Qfin and engine operational state quantity asengine rotational speed NE on one hand and target converted EGR ratioSRtgt on the other hand, and on the basis of an actual command injectionamount Qfin and an actual engine rotational speed NE. Further, an actualconverted EGR ratio SRact is calculated on the basis of expressions (3)and (4) shown below.

In the expressions (3) and (4), Ract represents an actual EGR ratio, andGn represents a flow rate of air (fresh air) that is actually suckedinto the cylinders of the engine (hereinafter referred to as an “actualair flow rate Gn” or a “detected air flow rate Gn”). The flow rate ofair (fresh air) is detected by an air flow meter. Further, Gcylrepresents a flow rate of gas that is actually sucked into the cylindersof the engine (i.e., flow rate of the entire gas=flow rate of freshair+flow rate of EGR gas). This flow rate (hereinafter referred to as a“cylinder inflow gas flow rate Gcyl”) is determined on the basis of amap defining a relationship between intake pipe pressure PM and intakeair temperature THA on one hand and gas flow rate Gcyl on the otherhand, and on the basis of an actually detected intake pipe pressure PMand an actually detected intake air temperature THA.

Further, an air excessiveness ratio λ used in the expression (3) iscalculated on the basis of the aforementioned expression (1). In thiscase, a fuel injection amount Q per unit time in the expression (1)cannot be measured directly and thus is calculated, for example, from anengine rotational speed NE and a command injection amount (a requiredinjection amount) Qfin, which is determined on the basis of anaccelerator operation amount Accp and the engine rotational speed NE.

[Expression 3]SRact=Rac/λ  (3)

[Expression 4]Ract=(Gcyl−Gn)/Gcyl  (4)

However, even if a drive signal has been delivered to an injector sothat the command injection amount Qfin of fuel is injected therefrom, anactual fuel injection amount may not coincide with the command injectionamount Qfin owing to differences among individual products of theinjector or changes in performance of the injector during use thereof.Hence, the fuel injection amount Q per unit time in the aforementionedexpression (1) may become imprecise, so that the air excessiveness ratioλ may become imprecise. Therefore, an actual converted EGR ratio SRact,which is based on the aforementioned expression (3), may not becalculated precisely. As a result, the true converted EGR ratio SRactdoes not coincide with the target converted EGR ratio SRtgt, and theintake-air oxygen concentration “x” cannot be made equal to a desiredvalue “x”. In some cases, therefore, an increase in discharge amount ofNOx is caused.

DISCLOSURE OF THE INVENTION

It is an object of the invention to provide an EGR control device andmethod for an internal combustion engine which is capable of ensuring adesired intake-air oxygen concentration “x” and thus reducing adischarge amount of NOx even in the case where a command injectionamount Qfin of fuel is not injected when a drive signal is delivered toan injector with a view to injecting the command injection amount Qfinof fuel therefrom.

The target converted EGR ratio SRtgt is determined, for example, on thebasis of a command injection amount Qfin and an engine rotational speedNE as engine operational state quantities. Thus, if a target EGR ratioRtgt is defined as a value Rtgt (Qfin, NE) that is determined on thebasis of a command injection amount Qfin and an engine rotational speedNE and if a target air excessiveness ratio Xtgt is likewise defined as avalue Xtgt (Qfin, NE) that is determined on the basis of a commandinjection amount Qfin and an engine rotational speed NE, a targetconverted EGR ratio SRtgt can be defined as a value that is determinedon the basis of an expression (5) shown below.

[Expression 5]SRtgt=Rtgt/λtgt  (5)

Further, if a target air flow rate is defined as a value Gntgt (Qfin,NE) that is determined on the basis of a command injection amount Qfinand an engine rotational speed NE, a target air excessiveness ratio λtgtcan be calculated from the aforementioned expression (1), using anexpression (6) shown below. An expression (7) shown below is obtainedfrom the expression (6) and the aforementioned expression (5). It is tobe noted in these expressions that k1 is a coefficient intended forcoincidence between units or the like.

[Expression 6]λtgt=k1×Gntgt/Qfin  (6)

[Expression 7]SRtgt=Rtgt×Qfin/k1×Gntgt  (7)

On the other hand, an actual converted EGR ratio SRact is expressed byan expression (8) shown below, on the basis of the aforementionedexpression (3) (SRact=Ract/λ) and the aforementioned expression (1)(λ=k×Gn/Q).

[Expression 8]SRact=Ract×Qfin/k1×Gn  (8)

A comparison between the aforementioned expression (7) and theaforementioned expression (8) reveals that the command injection amountQfin and the coefficient k1 exist on the right side of each of theexpressions and thus can be eliminated. Namely, in order to ensurecoincidence between a target converted EGR ratio SRtgt and an actualconverted EGR ratio SRact, it is appropriate that a post-conversiontarget converted EGR ratio SRhtgt expressed by an expression (9) shownbelow be equal to a post-conversion actual converted EGR ratio SRhactexpressed by an expression (10) shown below.

[Expression 9]SRhtgt=Rtgt/Gntgt  (9)

[Expression 10]SRhact=Ract/Gn  (10)

The foregoing description illustrates a principle adopted by the EGRcontrol unit for the internal combustion engine in accordance with theinvention. As is apparent from the aforementioned expressions (9) and(10), if the EGR control valve is controlled on the basis of a targetEGR ratio Rtgt, a target air flow rate Gntgt, an actual EGR ratio Ract,and an actual air flow rate Gn, a target converted EGR ratio can be madeto coincide with an actual converted EGR ratio SRact, without using acommand injection amount Qfin for calculations. As a result, a desiredintake-air oxygen concentration “x” can be obtained, whereby it becomespossible to effectively reduce a discharge amount of NOx.

The EGR control device for the internal combustion engine in accordancewith the invention, which is based on the concept described hitherto, isprovided with an EGR passage through which an exhaust passage and anintake passage of the internal combustion engine communicate with eachother, and with an EGR control valve across which the EGR passageextends and which controls a flow rate of EGR gas flowing from theexhaust passage to the intake passage. This EGR control device ischaracterized by comprising operational state quantity acquisitionmeans, target EGR ratio determination means, target air flow ratedetermination means, actual EGR ratio acquisition means, actual air flowrate acquisition means, and EGR ratio control means. The operationalstate quantity acquisition means acquires an operational state quantityof the engine. The target EGR ratio determination means determines, onthe basis of the detected operational state quantity, a target value ofan EGR ratio, namely, a ratio of a flow rate of EGR gas sucked by theengine to a flow rate of gas sucked by the engine, as a target EGRratio. The target air flow rate determination means determines, on thebasis of the detected operational state quantity, a target value of aflow rate of air sucked by the engine, as a target air flow rate. Theactual EGR ratio acquisition means acquires a true EGR ratio on thebasis of the detected operational state quantity, as an actual EGRratio. The actual air flow rate acquisition means acquires, on the basisof the detected operational state quantity, an actual flow rate of airsucked by the engine, as an actual air flow rate. The EGR ratio controlmeans controls an actual EGR ratio by controlling an opening of the EGRcontrol valve in accordance with the target EGR ratio, the target airflow rate, the actual EGR ratio, and the actual air flow rate.

In this case, it is preferable that the EGR ratio control means bedesigned to calculate, as a target converted EGR ratio, a valuecorresponding to a ratio of the target EGR ratio to the target air flowrate, to calculate, as an actual converted EGR ratio, a valuecorresponding to a ratio of the actual EGR ratio to the actual air flowrate, and to control an opening of the EGR control valve such that thetarget converted EGR ratio becomes equal to the actual converted EGRratio.

According to these constructions, as described above, a target convertedEGR ratio SRtgt can be made to coincide with an actual converted EGRratio SRact. As a result, the intake-air oxygen concentration “x”assumes a desired value, whereby it becomes possible to effectivelyreduce a discharge amount of NOx.

In addition, if each side of each of the aforementioned expressions (9)and (10) is multiplied by an actual air flow rate Gn, expressions (11)and (12) shown below are obtained.

[Expression 11]SRhtgt×Gn=Rtgt×Gn/Gntgt  (11)

[Expression 12]SRhact×Gn=Ract  (12)

As is apparent from the aforementioned expressions (11) and (12), inorder for the target converted EGR ratio SRtgt to coincide with theactual converted EGR ratio SRact (i.e., in order for a post-conversiontarget converted EGR ratio SRhtgt to coincide with a post-conversionactual converted EGR ratio SRhact), it is appropriate that the rightside of the expression (11) be equal to the right side of the expression(12) (i.e., the actual EGR ratio Ract). It is to be noted in the presentspecification that the left side SRhtgt×Gn of the aforementionedexpression (11) is referred to as a “control target EGR ratio Rctgt”.

In view of the foregoing, it is preferable that the EGR ratio controlmeans be designed to calculate, as a control target EGR ratio, a valueobtained by multiplying a target converted EGR ratio determined inaccordance with a ratio of the target EGR ratio to the target air flowrate by the actual air flow rate, and to control an opening of the EGRcontrol valve such that the control target EGR ratio becomes equal tothe actual EGR ratio.

This construction also makes it possible to make a target converted EGRratio SRtgt coincident with an actual converted EGR ratio SRact andhence to effectively reduce a discharge amount of NOx while ensuring adesired intake-air oxygen concentration “x”.

According to the EGR control device of the aforementioned invention, itis preferable that the target EGR ratio Rtgt be determined such that theintake-air oxygen concentration “x”, which greatly influences adischarge amount of NOx, assumes a desired value. However, if the targetEGR ratio Rtgt is thus determined, the EGR control valve is socontrolled as to ensure a predetermined EGR ratio even in a region wherethe actual air flow rate Gn is relatively low, for example, as in thecase where there is a delay in supercharging as in an initialacceleration period of an internal combustion engine equipped with asupercharger, or as in the case where there is a low atmosphericpressure. Therefore, the actual air flow rate Gn may become excessivelylow to the extent of increasing a generation amount of so-called smoke.

Thus, it is preferable that the target EGR ratio determination means bedesigned to determine a target EGR ratio for controlling intake-airoxygen concentration and intending to obtain such an intake-air oxygenconcentration as will suppress generation of nitrogen oxides, tocalculate a critical target EGR ratio for suppressing generation ofsmoke or particulate matters, and to determine the lower one of thetarget EGR ratio for controlling intake-air oxygen concentration and thecritical target EGR ratio as the target EGR ratio. Now, if the targetEGR ratio for controlling intake-air oxygen concentration and intendingto obtain such an intake-air oxygen concentration as will suppressgeneration of NOx is defined as a target EGR ratio R02tgt forcontrolling intake-air oxygen concentration while an air flow rate forobtaining this target EGR ratio R02tgt for controlling intake-air oxygenconcentration is denoted by Gn02, an expression (13) shown below isobtained.

[Expression 13]R02tgt=(Gcyl−Gn02)/Gcyl  (13)

Given an injection amount of an arbitrary constant, a minimumdischargeable amount (a permissible amount) of smoke (or particulatematters) is determined in advance, and a minimum air flow rate forpreventing a generation amount of smoke from reaching or exceeding theamount that has been determined for the injection amount of thearbitrary constant is defined as Gnmin. In this case, the amount ofsmoke reaches the permissible amount when the air flow rate Gn becomesextremely low. Thus, the air flow rate Gn02 for obtaining the target EGRratio R02tgt for controlling intake-air oxygen concentration for theconstant injection amount becomes higher than the minimum air flow rateGnmin. In this case, a critical target EGR ratio RSMtgt for suppressinggeneration of smoke (a target EGR ratio for suppressing generation ofsmoke) is expressed by an expression (14) shown below.

[Expression 14]RSMtgt=(Gcyl−Gnmin)/Gcyl  (14)

Then, a target EGR ratio Rtgt for controlling the EGR control valve (anEGR ratio) is finally determined by an expression (15) shown below. Inthe expression (15), min(α, β) represents a function of selecting thesmaller one of values α and β.

[Expression 15]Rtgt=min(R02tgt, RSMtgt)  (15)

As a result, if the target EGR ratio R02tgt for controlling intake-airoxygen concentration is lower than the critical target EGR ratio RSMtgt(R02tgt<RSMtgt), there is established a relationship Rtgt=R02tgtaccording to the aforementioned expression (15). By controlling the EGRcontrol valve, the actual air flow rate Gn becomes equal to Gn02(Gn=Gn02). Accordingly, an expression (16) shown below is obtained.

[Expression 16]Gnmin<Gn02=Gn  (16)

On the other hand, if the target EGR ratio R02tgt for controllingintake-air oxygen concentration is equal to or higher than the criticaltarget EGR ratio RSMtgt (R02tgt≦RSMtgt), there is established arelationship Rtgt=RSMtgt according to the aforementioned expression(15). By controlling the EGR control valve, the actual air flow rate Gnbecomes equal to the minimum air flow rate Gnmin. Accordingly, anexpression (17) shown below is obtained.

[Expression 17]Gnmin=Gn  (17)

Based on the foregoing description, if the lower one of the target EGRratio R02tgt for controlling intake-air oxygen concentration and thecritical target EGR ratio RSMtgt is determined as the target EGR ratioas in the construction mentioned above, the actual air flow rate Gnbecomes equal to or higher than the minimum air flow rate Gnmin.Therefore, the generation amount of smoke (or particulate matters) canbe inhibited from exceeding the permissible amount.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic structural view of an entire system in which anEGR control device in accordance with a first embodiment of theinvention is applied to a four-cylinder internal combustion engine (adiesel engine).

FIG. 2 is a flowchart showing a program that is executed by a CPU shownin FIG. 1.

FIG. 3 is a flowchart showing a program that is executed by a CPU of anEGR control device in accordance with a second embodiment of theinvention.

FIG. 4 is a flowchart showing a program that is executed by a CPU of anEGR control device in accordance with a third embodiment of theinvention.

FIG. 5 is a graph showing a relationship between intake-air oxygenconcentration and converted EGR ratio.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the respective embodiments of an EGR control device for aninternal combustion engine (a diesel engine) in accordance with theinvention will be described with reference to the drawings. FIG. 1 is aschematic structural view of an entire system in which the EGR controldevice (an exhaust gas purifying device for an internal combustionengine) in accordance with the first embodiment of the invention isapplied to a four-cylinder internal combustion engine (a diesel engine)10. This system includes an engine body 20 including a fuel supplyingsystem, an intake system 30 for introducing gas into combustion chambersformed respectively in cylinders of the engine body 20, an exhaustsystem 40 for discharging exhaust gas from the engine body 20, an EGRunit 50 for recirculating exhaust gas, and an electronic control unit60.

Each of fuel injection valves (injection valves or injectors) 21 isdisposed in an upper portion of a corresponding one of the cylinders ofthe engine body 20. The fuel injection valves 21 are connected to a fuelinjection pump 22 via a fuel line 23. The fuel injection pump 22 isconnected to a fuel tank (not shown). Thus, fuel that has beenpressurized up to an injection pressure is supplied to the fuelinjection valves 21 from the fuel injection pump 22. Also, the fuelinjection valves 21 are electrically connected to the electronic controlunit 60. Upon receiving a drive signal (a command signal correspondingto a command injection amount Qfin) from the electronic control unit 60,the fuel injection valves 21 are held open for a predetermined period.Thus, the pressurized fuel is injected into the combustion chambersformed respectively in the cylinders.

The intake system 30 includes an intake manifold 31, an intake pipe 32,a throttle valve 38, a throttle valve actuator 33 a, an intercooler 34,a compressor 35 a of a supercharger 35, and an air cleaner 36. Theintake manifold 31 is connected to the combustion chambers formedrespectively in the cylinders of the engine body 20. The intake pipe 32is connected to an upstream-side collective portion of the intakemanifold 31. The intake pipe 32 and the intake manifold 31 constitute anintake passage. The throttle valve 38 is rotatably held in the intakepipe 32. The throttle valve actuator 33 a rotatably drives the throttlevalve 33 in response to a drive signal transmitted from the electroniccontrol unit 60. The intercooler 34 and the compressor 35 a of thesupercharger 35 are disposed in this order along the intake pipe 32 in aregion upstream of the throttle valve 33. The air cleaner 36 is disposedat a tip end portion of the intake pipe 32.

The exhaust system 40 includes an exhaust manifold 41, an exhaust pipe42, a turbine 35 b of the supercharger 35, and a diesel particulatefilter (hereinafter referred to as a “DPNR”) 43. The exhaust manifold 41is connected to the cylinders of the engine body 20. The exhaust pipe 42is connected to a downstream-side collective portion of the exhaustmanifold 41. The turbine 35 b of the supercharger 35 is disposed in theexhaust pipe 42. The exhaust pipe 42 extends across the DPNR 43. Theexhaust manifold 41 and the exhaust pipe 42 constitute an exhaustpassage.

The DPNR 43 is provided with a filter 43 a that is made of a porousmaterial such as cordierite or the like. Particulates contained inexhaust gas flowing through the filter 43 a are collected on surfaces ofpores formed in the filter 43 a. The DPNR 43 includes alumina as acarrier. At least one material selected from an alkali metal such aspotassium K, sodium Na, lithium Li, and cesium Cs, an alkali earth metalsuch as barium Ba and calcium Ca, and a rare earth metal such aslanthanum La and yttrium Y is carried on the carrier. Platinum iscarried on the carrier as well. The DPNR 43 also functions as anocclusion-reduction-type NOx catalyst. After having absorbed NOx, theocclusion-reduction-type NOx catalyst discharges and reduces theabsorbed NOx.

The EGR unit 50 is provided with an exhaust gas recirculation pipe 51,an EGR control valve 52, and an EGR cooler 53. The exhaust gasrecirculation pipe 51 constitutes a passage (an EGR passage) throughwhich exhaust gas is recirculated. The exhaust gas recirculation pipe 51extends across the EGR valve 52. The exhaust passage communicates in aregion upstream of the turbine 35 b (the exhaust manifold 41) with theintake passage in a region downstream of the throttle valve 33 (theintake manifold 31), through the exhaust gas recirculation pipe 51. Inresponse to a drive signal transmitted from the electronic control unit60, the EGR control valve 52 changes an amount of exhaust gas to berecirculated (an exhaust gas recirculation amount or an EGR gas flowrate), and controls an EGR ratio as will be described later.

The electronic control unit 60 is a microcomputer composed of a CPU 61,a ROM 62, a RAM 63, a back-up RAM 64, an interface 65, and the like,which are interconnected by a bus. Programs executed by the CPU 61,tables (look-up tables and maps), constants, and the like are stored inthe ROM 62 in advance. The CPU 61 temporarily stores data into the RAM63 if the necessity arises. The back-up RAM 64 stores data when a powersource is on, and holds the stored data even while the power source isoff. The interface 65 includes an AD converter.

The interface 65 is connected to a hot-wire air flow meter 71, an intakeair temperature sensor 72, an intake pipe pressure sensor 73, an enginerotational speed sensor 74, and an accelerator opening sensor 75. Thehot-wire air flow meter 71, which is air flow rate (fresh air flow rate)measurement means, is disposed in the intake pipe 32. The intake airtemperature sensor 72 is provided in the intake passage at a positiondownstream of the throttle valve 33. The intake pipe pressure sensor 73is disposed in the intake passage at a position downstream of thethrottle valve 33 and downstream of a region to which the exhaust gasrecirculation pipe 51 is connected. Signals transmitted from thesesensors are supplied to the CPU 61. The interface 65 is connected to thefuel injection valves 21, the throttle valve actuator 33 a, and the EGRcontrol valve 52. In accordance with a command of the CPU 61, theinterface 65 delivers drive signals to those components.

The hot-wire air flow meter 71 measures a mass flow rate of intake air(an amount of intake air per unit time or an amount of fresh air perunit time) flowing through the intake passage, and generates a signal Gn(an air flow rate Gn) indicating the mass flow rate. The intake airtemperature sensor 72 detects a temperature of gas (i.e., an intake airtemperature) sucked into the cylinders (i.e., the combustion chambers)of the engine 10, and generates a signal THA indicating the intake airtemperature. The intake pipe pressure sensor 73 generates a signal PM(an intake pipe pressure PM) indicating a pressure in the intake passageat a position downstream of the throttle valve 33 and the EGR controlvalve 52.

The engine rotational speed sensor 74 detects a rotational speed of theengine 10, generates a signal indicating an engine rotational speed NE,and can detect an absolute crank angle of each of the cylinders. Theaccelerator opening sensor 75 detects an operation amount of anaccelerator pedal AP and generates a signal Accp indicating anaccelerator operation amount.

Next, operation of the EGR control device constructed as described abovewill be described. The CPU 61 of the electronic control unit 60repeatedly executes a program shown in the flowchart of FIG. 2 everytime a predetermined period elapses. Thus, at predetermined timings, theCPU 61 starts performing processings in a step 200. In a step 205, theCPU 61 fetches (acquires) operational state quantities of the enginesuch as an engine rotational speed NE, an accelerator operation amountAccp, an actual air flow rate Gn, an intake pipe pressure PM, an intakeair temperature THA, and the like, from the aforementioned varioussensors.

The CPU 61 then proceeds to a step 210 and determines a commandinjection amount Qfin (an amount of fuel to be injected or a requiredfuel injection amount) at that moment on the basis of a map defining arelationship between accelerator operation amount Accp and enginerotational speed NE as engine operational state quantities on one handand command injection amount Qfin on the other hand, and on the basis ofthe actual accelerator operation amount Accp fetched in the step 205 andthe actual engine rotational speed NE fetched in the step 205.

The CPU 61 then determines a target EGR ratio Rtgt on the basis of a mapdefining a relationship between command injection amount Qfin and enginerotational speed NE as engine operational state quantities on one handand target EGR ratio Rtgt on the other hand, and on the basis of theactual command injection amount Qfin acquired in the step 210 and theactual engine rotational speed NE fetched in the step 205. Then in astep 220, the CPU 61 determines a target air flow rate Gntgt on thebasis of a map defining a relationship between command injection amountQfin and engine rotational speed NE on one hand and target air flow rateGntgt on the other hand, and on the basis of the aforementioned commandinjection amount Qfin and the aforementioned actual engine rotationalspeed NE.

The CPU 61 then proceeds to a step 225. In this step, the CPU 61determines a (post-conversion) target converted EGR ratio SRhtgt fromthe target EGR ratio Rtgt acquired in the step 215 and the target airflow rate Gntgt acquired in the step 220, on the basis of theaforementioned expression (9). The target conversion EGR ratio SRhtgt isdetermined such that a predetermined intake-air oxygen concentration “x”for reducing a discharge amount of NOx is obtained. In other words, themaps used in the steps 215 and 220 are so determined as to obtain atarget converted EGR ratio SRhtgt, which is determined such that apredetermined intake-air oxygen concentration “x” for reducing adischarge amount of NOx is obtained. Then in a step 230, the CPU 61determines a cylinder inflow gas flow rate Gcyl on the basis of a mapdefining a relationship between intake pipe pressure PM and intake airtemperature THA as engine operational state quantities on one hand andcylinder inflow gas flow rate Gcyl on the other hand, and on the basisof the actual intake pipe pressure PM fetched in the step 205 and theactual intake air temperature THA fetched in the step 205.

Then in a step 235, the CPU 61 calculates and determines an actual EGRratio Ract from the aforementioned cylinder inflow gas flow rate Gcyl,the actual air flow rate Gn fetched in the step 205, and theaforementioned expression (4). In a step 240, the CPU 61 determines an(post-conversion) actual converted EGR ratio SRhact from the actual EGRratio Ract calculated in the step 240, the fetched actual air flow rateGn, and the aforementioned expression (10).

The CPU 61 then determines in a step 245 whether or not the actualconverted EGR ratio SRhact calculated in the step 240 is higher than thetarget converted EGR ratio SRhtgt calculated in the step 225. If theresult of the determination in the step 245 is “Yes”, the CPU 61proceeds to a step 250, causes the EGR control valve 52 to close by apredetermined opening so as to reduce an EGR ratio, and then proceeds toa step 265. On the other hand, if the result of the determination in thestep 245 is “No”, the CPU 61 proceeds to the step 255 and determineswhether or not the actual converted EGR ratio SRhact is lower than thetarget converted EGR ratio SRhtgt. If the result of the determination inthe step 255 is “Yes”, the CPU 61 proceeds to a step 260, causes the EGRcontrol valve 52 to open by a predetermined opening so as to increase anEGR ratio, and then proceeds to a step 265. If the result of thedetermination in the step 255 is “No”, the CPU 61 directly proceeds tothe step 265.

The CPU 61 then determines in the step 265 whether or not fuel is to beinjected at the moment. If fuel is to be injected at the moment, the CPU61 ensures that the command injection amount Qfin of fuel calculated inthe step 210 is injected from a corresponding one of the fuel injectionvalves 21 of at least one of the cylinders from which fuel is to beinjected, and temporarily terminates the present routine in a step 295.If the result of the determination in the step 265 is “No”, the CPU 61directly proceeds to the step 295 and temporarily terminates the presentroutine.

As described hitherto, according to the first embodiment of theinvention, an opening of the EGR control valve 52 is controlled suchthat an actual converted EGR ratio SRhact coincides with a targetconverted EGR ratio SRhtgt, whereby an EGR ratio is changed. In thiscase, the actual converted EGR ratio SRhact and the target converted EGRratio SRhtgt are calculated without using a command injection amountQfin. Therefore, even if the command injection amount Qfin of fuel isnot injected from the fuel injection valves 21, a true value of theactual converted EGR ratio SRhact is precisely calculated. Thus, theintake-air oxygen concentration “x” can be so controlled with highprecision as to become equal to a desired value. Therefore, it ispossible to reduce a discharge amount of NOx.

Next, the EGR control device in accordance with the second embodiment ofthe invention will be described. This EGR control device is differentfrom the EGR control device of the aforementioned first embodiment onlyin that the CPU 61 executes a program shown in FIG. 3 instead of theprogram shown in FIG. 2. Accordingly, the following description willfocus on the difference.

The CPU 61 repeatedly executes the program shown in FIG. 3 every time apredetermined period elapses. Thus, at predetermined timings, the CPU 61performs processings in steps 305 to 320 following a step 300, anddetermines a command injection amount Qfin, a target EGR ratio Rtgt, anda target air flow rate Gntgt. The steps 305 to 320 are the same as theaforementioned steps 205 to 220 respectively and thus will not bedescribed in detail.

The CPU 61 then proceeds to a step 325 and determines a control targetEGR ratio Rctgt on the basis of the aforementioned expression (11). TheCPU 61 then proceeds to a step 330 where the same processing as in thestep 230 is performed, and determines a cylinder inflow gas flow rateGcyl. Also, the CPU 61 calculates an actual EGR ratio Ract in a step 335where the same processing as in the step 235 is performed.

The CPU 61 then determines in a step 340 whether or not the actual EGRratio Ract calculated in the step 335 is higher than the control targetEGR ratio Rctgt calculated in the step 325. If the result of thedetermination in the step 340 is “Yes”, the CPU 11 proceeds to a step345, causes the EGR control valve 52 to close by a predetermined openingso as to reduce an EGR ratio, and proceeds to a step 360. On the otherhand, if the result of the step 340 is “No”, the CPU 61 proceeds to astep 350 and determines whether or not the actual EGR ratio Ract islower than the control target EGR ratio Rctgt. If the result of the step350 is “Yes”, the CPU 61 proceeds to a step 355, causes the EGR controlvalve 52 to open by a predetermined opening so as to increase an EGRratio, and proceeds to the step 360. If the result of the determinationin the step 350 is “No”, the CPU 61 directly proceeds to the step 360.

The CPU 61 then determines in the step 360 whether or not fuel is to beinjected at the moment. If fuel is to be injected at the moment, the CPU61 ensures in a step 365 that the command injection amount Qfin of fuelcalculated in the step 310 is injected from a corresponding one of thefuel injection valves 21 of at least one of the cylinders from whichfuel is to be injected, and temporarily terminates the present routinein a step 395. If the result of the determination in the step 360 is“No”, the CPU 61 directly proceeds to the step 395 and temporarilyterminates the present routine.

As described hitherto, according to the second embodiment of theinvention, an opening of the EGR control valve 52 is controlled suchthat an actual EGR ratio Ract coincides with a control target EGR ratioRctgt, whereby an EGR ratio is changed. In this case, the actual EGRratio Ract and the control target EGR ratio Rctgt are calculated withoutusing a command injection amount Qfin. Therefore, even if the commandinjection amount Qfin of fuel is not injected from the fuel injectionvalves 21, a true value of the EGR ratio Ract is made to coincideprecisely with the control target EGR ratio Rctgt. As a result, theintake-air oxygen concentration “x” is so controlled with high precisionas to become equal to a desired value. Therefore, it is possible toreduce a discharge amount of NOx.

The target converted EGR ratio SRhtgt and the actual converted EGR ratioSRhact that are used in the control of the first embodiment are hard torecognize as physical quantities, whereas the control target EGR ratioand the actual EGR ratio that are used in the second embodiment arequantities that are physically easy to grasp. Namely, the control of thesecond embodiment is designed as follows. In order to make an intake-airoxygen concentration “x” constant for a target EGR ratio that has beendetermined in a circumstance with a constant engine rotational speed anda constant injection amount (i.e., with a constant cylinder inflow gasflow rate Gcyl), an EGR ratio is reduced in the case where an actual airflow rate is lower than a target air flow rate in a similar circumstancewith a constant engine rotational speed and a constant injection amount,whereas an EGR ratio is increased in the case where an actual air flowrate is higher than the target air flow rate. Therefore, various controlconstants of the engine can be adapted easily.

Next, the EGR control device in accordance with the third embodiment ofthe invention will be described. This EGR control device is differentfrom the EGR control device of the second embodiment only in that theCPU 61 performs processings in steps 405 to 420 shown in the flowchartof FIG. 4 instead of the processing in the step 315 of FIG. 3, which isperformed by the CPU 61 of the aforementioned second embodiment.Accordingly, the following description will focus on the difference.

After having finished the processing in the step 310 shown in FIG. 3 ata predetermined timing, the CPU 61 in accordance with the thirdembodiment proceeds to a step 405 shown in FIG. 4, and determines atarget EGR ratio R02tgt for controlling intake-air oxygen concentrationand obtaining such an intake-air oxygen concentration as will suppressgeneration of NOx in the step 405. The target EGR ratio R02tgt forcontrolling intake-air oxygen concentration is determined from a mapdefining a relationship between command injection amount Qfin and enginerotational speed NE on one hand and target EGR ratio R02tgt on the otherhand, and from a current command injection amount Qfin calculated in thestep 310 and a current actual engine rotational speed NE.

The CPU 61 then proceeds to a step 410 and determines a minimum value (aminimum air flow rate) Gnmin of an air flow rate required for making anamount of smoke or particulate matters equal to or smaller than apredetermined amount (a predetermined permissible amount) for thecommand injection amount Qfin, on the basis of the command injectionamount Qfin. The CPU 61 then calculates a target EGR ratio RSMtgt forsuppressing generation of smoke (or for suppressing generation ofparticulate matters) according to the aforementioned expression (14) ina step 415. In a step 420, the CPU 61 determines and adopts the lowerone of the target EGR ratio (a critical target EGR ratio) R02tgt forcontrolling intake-air oxygen concentration and the target EGR ratioRSMtgt for suppressing generation of smoke, as a final target EGR ratioRtgt.

The CPU 61 thereafter performs the processings in the steps 320 to 395of FIG. 3, and controls the EGR control valve 52 such that an actual EGRratio Ract coincides with a control target EGR ratio Rctgt (see the step325) that has been determined in accordance with the final target EGRratio Rtgt.

Consequently, as described above, an actual air flow rate Gn is equal toor higher than a minimum air flow rate Gnmin that is determined inaccordance with a command injection amount Qfin. Therefore, it ispossible to reduce a discharge amount of NOx while reducing an amount ofsmoke to a permissible amount or less.

As described hitherto, the EGR control device for the internalcombustion engine in accordance with each of the embodiments of the EGRcontrol device makes it possible to reduce a discharge amount of NOx. Itis to be noted herein that the invention is not limited to theaforementioned embodiments and that various modification examples can beadopted within the scope of the invention. For instance, the thirdembodiment can be used in combination with the first embodiment. Thatis, it is possible to adopt a construction in which the processings inthe steps 405 to 420 of FIG. 4 are performed instead of the processingin the step 215 of FIG. 2.

1. An EGR control device for an internal combustion engine comprising:an EGR passage through which an exhaust passage and an intake passage ofthe internal combustion engine communicate with each other; an EGRcontrol valve across which the EGR passage extends and which controls aflow rate of EGR gas flowing from the exhaust passage to the intakepassage; and an EGR controller that: acquires an operational statequantity of the engine; determines, on the basis of the detectedoperational state quantity, a target value of an EGR ratio, namely, aratio of a flow rate of EGR gas sucked by the engine to a flow rate ofgas sucked by the engine, as a target EGR ratio; determines, on thebasis of the detected operational state quantity, a target value of aflow rate of air sucked by the engine, as a target air flow rate;acquires a true EGR ratio on the basis of the detected operational statequantity, as an actual EGR ratio; acquires, on the basis of the detectedoperational state quantity, an actual flow rate of air sucked by theengine, as an actual air flow rate; and defines a target converted EGRratio as a value corresponding to a ratio of the target EGR ratio to atarget excessiveness ratio as being a ratio of the target air flow rateto a command injection amount, and an actual converted EGR ratio as avalue corresponding to a ratio of the actual EGR ratio to an actualexcessiveness ratio as being a ratio of the actual air flow rate to thecommand injection amount, and controls an actual EGR ratio bycontrolling an opening of the EGR control valve in accordance with theconverted actual EGR ratio and the target converted EGR ratio.
 2. TheEGR control device according to claim 1, wherein the EGR calculates, asa control target EGR ratio, a value obtained by multiplying the targetconverted EGR ratio by the actual air flow rate, and controls an openingof the EGR control valve such that the control target EGR ratio becomesequal to the actual EGR ratio.
 3. The EGR control device according toclaim 1, wherein the controller determines a target EGR ratio from acommand fuel injection amount and an engine rotational speed.
 4. The EGRcontrol device according to claim 1, wherein the controller determines atarget air flow rate from a command fuel injection amount and an enginerotational speed.
 5. The EGR control device according to claim 1,wherein the controller determines an actual EGR ratio from a cylinderinflow gas flow rate and an actual air flow rate.
 6. The EGR controldevice according to claim 1, wherein the controller acquires an actualair flow rate from an air flow meter.
 7. The EGR control deviceaccording to claim 1, wherein the controller determines a target EGRratio for controlling intake-air oxygen concentration and obtaining suchan intake-air oxygen concentration as will suppress generation ofnitrogen oxides, calculates a critical target EGR ratio for suppressinggeneration of smoke or particulate matters, and determines the lower oneof the target EGR ratio for controlling intake-air oxygen concentrationand the critical target EGR ratio as the target EGR ratio.
 8. The EGRcontrol device according to claim 7, wherein the EGR ratio forcontrolling intake-air oxygen concentration is determined from a commandfuel injection amount and an engine rotational speed.
 9. The EGR controldevice according to claim 7, wherein the critical target EGR ratio isdetermined from a cylinder inflow gas amount and a smoke criticalminimum air flow rate.
 10. An EGR control method for an internalcombustion engine provided with an EGR passage through which an exhaustpassage and an intake passage of the internal combustion enginecommunicate with each other, and with an EGR control valve across whichthe EGR passage extends and which controls a flow rate of EGR gasflowing from the exhaust passage to the intake passage, comprising thesteps of: acquiring an operational state quantity of the engine;determining, on the basis of the detected operational state quantity, atarget value of an EGR ratio, namely, a ratio of a flow rate of EGR gassucked by the engine to a flow rate of gas sucked by the engine, as atarget EGR ratio; determining, on the basis of the detected operationalstate quantity, a target value of a flow rate of air sucked by theengine, as a target air flow rate; acquiring a true EGR ratio on thebasis of the detected operational state quantity, as an actual EGRratio; acquiring, on the basis of the detected operational statequantity, an actual flow rate of air sucked by the engine, as an actualair flow rate; and defining a target converted EGR ratio as a valuecorresponding to a ratio of the target EGR ratio to a targetexcessiveness ratio as being a ratio of the actual air flow rate to acommand injection amount, and an actual converted EGR ratio as a valuecorresponding to a ratio of the actual EGR ratio to an actualexcessiveness ratio as being a ratio of the actual air flow rate to thecommand injection amount, and controlling an actual EGR ratio bycontrolling an opening of the EGR control valve in accordance with theactual converted EGR ratio and the target converted EGR ratio.
 11. TheEGR control method according to claim 10, wherein the EGR ratio controlcalculates, as a control target EGR ratio, a value obtained bymultiplying the target converted EGR ratio by the actual air flow rate,and controls an opening of the EGR control valve such that the controltarget EGR ratio becomes equal to the actual EGR ratio.
 12. The EGRcontrol method according to claim 10, wherein the determination of thetarget EGR ratio determines a target EGR ratio from a command fuelinjection amount and an engine rotational speed.
 13. The EGR controlmethod according to claim 10, wherein the determination of the targetair flow rate determines a target air flow rate from a command fuelinjection amount and an engine rotational speed.
 14. The EGR controlmethod according to claim 10, wherein the acquisition of the actual EGRratio determines an actual EGR ratio from a cylinder inflow gas flowrate and an actual air flow rate.
 15. The EGR control method accordingto claim 10, wherein the acquisition of the actual air flow rateacquires an actual air flow rate from an air flow meter.
 16. The EGRcontrol method according to claim 10, wherein the determination of thetarget EGR ratio determines a target EGR ratio for controllingintake-air oxygen concentration and obtaining such an intake-air oxygenconcentration as will suppress generation of nitrogen oxides, calculatesa critical target EGR ratio for suppressing generation of smoke orparticulate matters, and determines the lower one of the target EGRratio for controlling intake-air oxygen concentration and the criticaltarget EGR ratio as the target EGR ratio.
 17. The EGR control methodaccording to claim 16, wherein the EGR ratio for controlling intake-airoxygen concentration is determined from a command fuel injection amountand an engine rotational speed.
 18. The EGR control method according toclaim 16, wherein the critical target EGR ratio is determined from acylinder inflow gas amount and a smoke critical minimum air flow rate.